opaque to the text until the cursor moves over them or I select
the region

with the mouse.

Can you reproduce starting with Emacs -Q?

I can. After startup, select a row of text, click onto another
application, then back. I don't know if this was there before the
patch.

It appears that -Q is not respected: org.gnu.Emacs.plist is still read.
That's why I couldn't reproduce this, but I can see the bars now.
This only occurs when cursor-type is set to t.
D
PS.: Getting these. Presumably unrelated.